1. Field of the Invention
The present invention relates to a method and an apparatus for correcting errors of read-out data read out from a medium in which data is stored.
2. Description of Related Art
As storage medium for storing information such as documents, images and sound, there have been storage media (media) such as magnetic disks (typically hard disks HD), digital video disks (DVD), magneto-optic disks (MO), compact disks (CD) and laser disks (LD). When data is reproduced from the media such as DVDs and CDs, errors occur in read-out data owing to changes of reflectance of light, interference between waves, noises and the like. Among the errors, the probability of occurrence of bit shift of one bit is the highest.
FIG. 1 is a block diagram showing a flow of a conventional device for reproducing the read-out data which is read out from conventional media. A readout signal from media 1 is recovered by a read channel 2, and converted to readout data composed of a bit string. The read-out data after the conversion is sent to a mark detector 3. The mark detector 3 detects xe2x80x9cSYNCxe2x80x9d and a data area in the read-out data. The detected data is decoded in a decoder 4 which includes conversion table 5.
This conventional decoder 4 does not detect whether or not the read-out data (bit string) violates a run length limited (RLL) rule. Furthermore, the decoder 4 does not perform corrections of errors in the read-out data. Conventionally ECC (Error Correction Code) corrects errors in the read-out data. In this case, the number of errors correctable depends on the maximum processing capability of the ECC and errors exceeding the limit of the ECC cannot be corrected. Therefore, there can be data errors which are not corrected by the ECC.
The object of the present invention is to provide a method and an apparatus, which are capable of correcting errors of read-out data from a data storage medium, especially bit shift errors, regardless of whatever additional ECC may exist.
Another object of the present invention is to provide a novel decoding system capable of correcting errors of readout data while concurrently decoding the read-out data without degrading its performance.
Still another object of the present invention is to provide a storage device capable of enhancing a signal to noise ratio and decreasing a load on the ECC by executing an error correction by a read channel decoder before conventional ECC processing.
The correction apparatus includes a gray bit detection circuit which flags a bit in a bit cell with a phase shift exceeding a threshold and determines whether a phase error between an intersection of the bit cell and the center of a previous bit cell or a phase error between an intersection of the bit cell and the center of a next bit cell is a smaller phase error. An RLL error correction circuit and a table containing valid bit combinations for the read-out data may be used in combination with the gray bit detector to correct errors on-the-fly without degrading performance. An advantage of the invention is that it allows correction of errors without regard to conventional ECC and its maximum number of errors.